The invention relates to a probe for detecting a dielectric medium defined by a face and comprising at least one transmitter/receiver antenna located near the face, a signal processor for receiving signals from the antenna and generating data representative of the structure of the dielectric medium, and a dielectric material located between the at least one transmitter/receiver antenna and the boundary surface.
In particular, although not exclusively, the invention relates to a probe for detecting voids in the soil around a pipe, such as e.g. a sewer pipe.
By the term xe2x80x9cdetectedxe2x80x9d, it is in the above sense to be understood that the dielectric face and the dielectric medium are inspected, measured, analysed and described, the latter by means of e.g. data and/or images.
It is well known that voids can be formed around a sewer pipe. Formation of voids typically arise by sediment being washed into a leaky pipe because this pipe is functioning as drain. In connection with heavy rain, the water pressure in the pipe will often exceed the water pressure in the surrounding sediment whereby water is flowing out of the pipe. On its way back into the pipe, the water will bring along sediment.
A void, once formed, is in time likely to grow resulting in the fact that depressions are formed in the overlying ground. If the ground surface is covered with asphalt or concrete, the void can gradually assume a substantial size before the damage really appears.
Furthermore, the voids are often popular habitats and breeding grounds for rats. However, the rats are not efficiently exterminated without knowledge of the existence and location of the voids.
The voids are difficult and often impossible to reveal by detection from the ground surface. The problem cannot be solved by means of a video camera guided through the pipe either. By means of a video camera, it is possible to observe the inside of the pipe but not areas outside the pipe. For example, a bad and leaking joint cannot always be seen with a video camera.
Instead, these voids are therefore attempted to be revealed and localized by detection from within the pipe using probes designed for this specific purpose.
Such a probe is known from EP 0 816 872 A1. In this case, a number of antennas are placed along the periphery of the probe. A signal processor is furthermore part of the probe for generating three-dimensional images of the signals from the antennas.
The known probe operates with a radar which is able to transmit signals through the wall of the pipe. On its way through the pipe, the probe is thus emitting radar signals into the surrounding soil at short time intervals and detecting the returned radar signals. The signal processor converts the signals into data representative of the structure in the soil around the location in the pipe where the probe or rather its antennas are at a given moment.
If the soil around the pipe is homogeneous and without other objects, the data recorded during the travel of the probe through the pipe will assume an even and regular character. Conversely, deviations are signs that there are voids or objects in the soil of divergent electric properties (permittivity). Normally, the voids and objects can be distinguished from one another on the diversity of the data.
The known probe makes use of several closely set antennas. Thereby, the advantage is obtained in that it is only necessary to guide the probe through the pipe one time in order to at least essentially detect abnormal conditions in the ground around the pipe.
However, the number of antennas is limited by the modest space left when many antennas in this way must be placed closely next to each other along the periphery of the probe. This is especially a problem in case of small pipe dimensions.
Another disadvantage is that it is not possible freely to choose the location of the measuring profile or the closeness between the profiles by means of this probe. Moreover, the measurements are limited to only be able to take place along the pipe as the probe cannot be made to rotate in the pipe on a given location for example in order to check a joint and possible corrosion in the top and/or bottom of the pipe.
Theoretically, the antennas can be located in direct contact with the wall of the pipe. However, since a pipe normally does not have the precise same inside diameter in all cross sections, such a positioning is however not always possible in practice.
However, it is possible to compensate for the varying and imprecise inside diameter of a given pipe by letting the antennas be spring-loaded and dragging them along the inside face of the pipe. Since pipes of e.g. concrete often have a rough and uneven inside face, this solution is only suitable for minor inspections where the inevitable wearing of the antennas is not of great importance.
In practice, there must therefore be a gap conventionally filled with air between the transmitter/receiver antennas and the inside face of the pipe. However, the existence of such a gap reduces the effect of the signals penetrating into the ground around the pipe. Another detrimental effect is that undesirable reflections from the wall of the pipe are generated in addition to the desired reflections.
A preferred antenna type would be a high-frequency shielded dipole antenna. These antennas are normally arranged for direct connection with the base, that is the distance between the medium being checked and the antenna must be as short as possible. Even if the antenna is spring-loaded, it will be necessary to have a certain distance between the antenna and e.g. the inside face of the sewer pipe due to oval pipes, staggered joints etc. as the probe otherwise could get stuck too frequently.
However, an increased distance between the inside face of the sewer pipe and the antenna increases the surface reflections. Apart from a smaller effect in the part of the signal penetrating the pipe and into the ground, it also has the effect of reflections being generated inside the pipe from the probe and the inside face of the pipe. These undesired reflections can be removed during subsequent signal processing to some extent but they still degrade the final measuring results.
GB 2 166 599 A discloses an electromagnetic detector system for detecting the layers surrounding a horizontal borehole. Above an antenna which is part of the system, a dielectric material is located with a dielectric constant mainly corresponding to the dielectric constant of the surrounding layers.
U.S. Pat. No. 5,138,263 discloses a drill string for measuring the electric properties of an earth formation surrounding the borehole. For this purpose, a transmitting antenna is used for generating electromagnetic waves and at least two receiving antennas for receiving the reflected electromagnetic waves. The antennas are covered by an elastomeric layer which is located in a groove in the drill string. The layer is serving for protecting the antennas against shocks and abrasive contaminants.
In both of these patent documents, transmission of the signals takes place via a gap between the respective probe and the boundary surface resulting in the aforesaid disadvantages.
Furthermore, German Patent Publication DE 40 17 238 A1 discloses a method and a device for detecting e.g. a concrete pipe and its surroundings. In this case, an antenna is located rotatably on a shaft mounted on a vehicle only shown schematically. It is mentioned that the axis of the shaft coincides with the axis of the pipe but the publication does not mention any means for achieving this. By means of the arrangement shown only schematically in the drawing, it is however not possible to obtain a fairly regular, constant distance between the antenna and the inside face of the pipe, and the antenna must therefore be located at an adequate distance from the inside face of the pipe.
The present invention now provides a device and method that overcomes the problems of the prior art.
The present invention provides a system that provides an improved connection between the antennas of a detection probe probe and a dielectric boundary surface. The probe comprises at least one transmitter/receiver antenna located near the surface, a signal processor for receiving signals from the antenna and generating data representative of the structure of the dielectric medium, and a dielectric material located between the at least one transmitter/receiver antenna and the boundary surface.
The probe of the invention is arranged to be able to utilize the capacity of the antennas optimally and which is selectively able to detect the surroundings of a given pipe. Also, the preferred probes of the invention have a simple and inexpensive design.
The novel and unique features according to the invention are achieved by configuring the dielectric material of the probe as a flexible cushion in contact with or near the boundary surface of the dielectric material while detection takes place. An improved connection between e.g. the inside face of a sewer pipe and an antenna located at a distance from this inside face can be achieved when the cushion is made of a material that has a dielectric number that is higher than that of the air and smaller than that of the soil around the pipe.
The cushion can advantageously be laminated so that a stepped or gradual increase of the dielectric number out towards the surroundings is obtained. The cushion, which is mounted on or around the antenna, has the effect that as much as possible of the transmitted signal penetrates out into the surrounding soil. At the same time, the amount of undesired reflexes from the opposite side of the pipe is reduced. In addition, the cushion has the advantageous effect that the antenna is spared destructive shocks and vibrations, and that the probe is not likely to get stuck in the pipe.
In a simple embodiment, the cushion can be a watertight bag that contains a mixture of air and water. The desired dielectric number is achieved by mixing air and water in adequate proportion. This bag can typically be made of a suitable, impermeable plastic or rubber material to ensure that the contents of the bag retains the desired amount of water and that the dielectric number thereby maintains at the predetermined value.
Especially along the attrition face towards the often rough and uneven pipe wall, the bag can be reinforced with an abrasion-proof material of e.g. woven nylon. Alternatively, a laminate with e.g. polyethylene on the inside and nylon on the outside can be used.
In an advantageous embodiment, a material can be provided in the bag. Advantageously, this material is a water-absorbing foam of plastic or rubber having an open cell structure. This foam can be in form of a sponge or it can be particular. The water-absorbing material or foam is made to absorb so much water that the desired dielectric number is achieved.
An even and uniform distribution of the water in the water-absorbing material is achieved when a fine-grained foam having good capillary properties is used. In order to increase viscosity and give the water-absorbing material or foam an adequate elasticity, the water can furthermore be added a polymer which can give the water phase an advantageous gelatinous consistency if desired.
When the foam with the open cell structure is particular, it can be mixed with particular foam having a closed cell structure. The foam particles having the open cell structure are made to absorb water, and the two kinds of particles, which have to be small in relation to the applied wavelength, are mixed in a proportion corresponding to the desired dielectric number. Where the cushion is used for dampening radar signals, the water phase can advantageously consist of a saline solution.
In some cases it would be desirable to be able to vary the dielectric number of a given cushion. For this purpose the probe can comprise means for variably be able to compress the flexible cushion. Thereby, the air-liquid phase ratio in the bag is altered and thus the dielectric number. This solution is suitable for e.g. inspecting a road surfacing or concrete construction.
As it appears, the flexible cushion according to the invention advantageously fills the gap between the antenna and the inside face of the pipe.
In practice, for example the cross section of the sewer pipes however differs more or less from a completely circular form. Furthermore, the pipes will usually have an uneven inside face and sometimes projecting edges at the joints.
If the probe is guided forward in the pipe in the conventional way described for example in German Patent Publication DE 40 17 238 A1, where the antenna is located on a shaft which can rotate about an axis, there is therefore a risk that the flexible cushion in some situations could lose contact with the inside face of the pipe so that the desired, optimal effect is not achieved. In other cases, the flexible cushion can be damaged by jamming against the inside face of the pipe or being more or less torn on a projecting or sharp edge.
According to the invention, by journaling the shaft in two bearings located on each their movable support one of which can be an automotive tractor and the other a carriage, the above-mentioned imperfections of the pipe are equalized. This carriage controls the shaft whereas the tractor pulls or pushes the carriage. The two independent supports ensure that the shaft is always centred in the best possible way in the pipe and that each antenna therefore is secured a mainly constant distance to the inside face of the pipe even if the cross sections and surfaces that the antenna encounters on its way through the pipe vary to a greater and smaller extent. This equalizing effect is especially effective when the distance between the supports of the shaft is relatively large.
By means of the above equalizing method according to the invention, two very important advantages are obtained namely that the antennas and cushions will be operating mainly under the same conditions in all angular positions and that the cushions on the whole are spared being damaged. Having this design, the probe is able to detect the soil around a pipe with merely one directional antenna in an area of 360xc2x0 with random measuring density.
In order to further ensure the correct positioning of the antenna in the pipe, a balance weight can be mounted on the shaft for counterbalancing the weight of the antenna. However, this balancing is not quite effective when the pipe is filled more or less with sewage water which affects the balance weight with a variable buoyant power. According to the invention, the shaft can therefore instead be divided into two shaft parts which are arranged to rotate in different directions at the same speed, at the same time a first antenna can be placed on one of the shaft parts and a second antenna on the second shaft, and the two antennas can, in a specific angular position of each of the two shaft parts, be symmetrically placed around a vertical plane comprising the axis of rotation. Thereby, the two antennas will balance each other out in each angular position even if the pipe is filled more or less with sewage water.
When both a tractor and a carriage having each their bearing of the shaft are used for supporting the shaft, the entire probe will be so long that it cannot or only with difficulty can be guided down into a pipe via a well without being articulated. According to the invention, this articulation can advantageously be made by means of at least one horizontally oriented swivel bearing and at least one vertically oriented swivel bearing.
With a view to absorb or dampen undesired reflexes in the pipe, a material can be fitted on the inside of the antenna for absorbing signals reflected from the wall of the pipe. Furthermore, a material can be fitted around the shaft carrying the antenna for absorbing reflected radar signals inside the pipe. For this purpose any kind of material can be used that is well suited for absorbing the reflected radar signals at the respective frequencies.
In a preferred embodiment, a cushion is used which is of the same type as the one described above, but with the difference that the water is replaced by a saline solution and that the dielectric number is kept at a value adequately low to ensure that the reflection from the cushion surface is the least possible and that the penetration and absorption of the signals the greatest possible.
The design of the probe permits accurate measurement of chosen cross sections of a pipe. This kind of measurement can e.g. serve for localization of damages at a joint, measurement of variations with regard to corrosion, erosion in a concrete pipe or localization of a service line behind a sleeve lining.
The probe can moreover have several antennas located at mutual angular distances on the shaft of the probe. Thereby, the number of run-throughs for measurements can be reduced considerably. Furthermore, the antennas can be displaced in relation to each other along the axis of rotation. As the probe is operating with one or just a small number of antennas, it is moreover simple and inexpensive to manufacture.